lib: Check for http_proxy being the empty string

On at least one user's computer, g_getenv("http_proxy") returns the
empty string, so check for that and treat it as no proxy rather than
printing a warning.

See https://github.com/flatpak/flatpak/issues/2790

Closes: #1835
Approved by: cgwalters

src/libostree/ostree-repo-pull.c

@@ -3034,7 +3034,7 @@ _ostree_repo_remote_new_fetcher (OstreeRepo  *self,
                                         &http_proxy, error))
       goto out;
 
-    if (http_proxy != NULL)
+    if (http_proxy != NULL && http_proxy[0] != '\0')
       _ostree_fetcher_set_proxy (fetcher, http_proxy);
   }